A conventional power supply apparatus using a power factor corrector and a quasi-resonant flyback converter converts an AC (alternating current) input voltage into a DC (direct current) output voltage. The output voltage is stabilized at a selected one of multiple predetermined voltage values. A current provided by the conventional power supply apparatus is dependent on loading of the same. The conventional power supply apparatus cannot have relatively high conversion efficiency under various specific settings of the output voltage and the loading since the power supply apparatus is normally well-designed under certain output voltage.